A gas-phase catalytic oxidation reaction using a solid catalyst is being performed on a commercial scale. As the gas-phase catalytic oxidation reaction, for example, a production of acrolein or acrylic acid from propylene and a production of methacrolein or methacrylic acid from isobutylene or tertiary butyl alcohol are exemplified.
These gas-phase catalytic oxidation reactions use molecular oxygen and synthesize a useful target compound by stopping a reaction at an intermediate oxidation state. For example, it is possible to synthesize acrolein from one mole of propylene and one mole of oxygen and to synthesize acrylic acid from one mole of acrolein and a half mole of oxygen.
However, in such an oxidation reaction, there occurs a decomposition reaction or an oxidation reaction simultaneously or successively as well as a reaction for obtaining a target product. As a result, a byproduct such as carbon dioxide, which is the most oxidized state, and the like may be generated in some cases.
Under these circumstances, because gas-phase catalytic oxidation reactions are complicated reactions, a manufacturing method by which a target product can be synthesized in a high yield is being investigated among such reactions.
A temperature condition with which a target product can be obtained in a high yield by stopping an oxidation reaction at an intermediate stage is in a narrow range. Usually, in the case that the temperature becomes higher than the optimum range, an amount of an oxidized decomposition product such as acetic acid, carbon monoxide or carbon dioxide is increased and consequently, the yield is lowered. Although an oxidation reaction in which a target product is produced is an exothermic reaction and accompanies a large heat release, the heat of reaction of these side reactions is still larger and when a rate of the side reactions becomes large, the overall heat of reaction becomes still larger. Further, because a reaction rate increases exponentially with a temperature, the side reactions may cause a runaway reaction. Therefore, when an oxidation reaction is performed in a fixed-bed multitubular reactor, it is required to define exactly a quality of a catalyst, a method of packing the catalyst or an operating condition to prevent the temperature from exceeding the optimum condition.
For example, a method for improving a temperature distribution in a reaction tube is being proposed as disclosed in Japanese Patent Application, First Publication No. Hei 6-192144.
In this literature, when producing methacrolein from isobutylene or tertiary butyl alcohol and molecular oxygen as raw materials by a gas-phase catalytic oxidation reaction, a method in which a catalyst powder supported on a carrier inactive to the reaction is used as the catalyst and a supported amount of the catalyst powder is increased gradually from an inlet part to an outlet part of a reaction tube by dividing a longitudinal direction of the reaction tube into a plurality of sections is disclosed.
Further, a method of promoting a removal of heat by increasing a circulating amount of a heat medium outside a reaction tube and a method of monitoring precisely the temperature in a reaction tube and the other methods are proposed as a method for performing a gas-phase catalytic oxidation reaction stably.
For example, Japanese Patent Application, First Publication No. 2001-139499 discloses a method of circulating a heat medium into a shell side of a reactor through a circulating devise in a fixed-bed multitubular reactor and suppressing an increase in a temperature in a reaction tube, wherein a part of a heat medium drawn out of the shell side of the reactor is heat exchanged and the heat-exchanged heat medium is returned to the shell side of the reactor, thereby controlling a temperature difference between the heat medium drawn out and the one introduced in at a range from 15 to 150° C.
Further, Japanese Patent Application, First Publication No. Hei 8-92147 discloses a method of suppressing a temperature of a catalyst layer, wherein when propylene is oxidized to acrolein with a gas-phase catalytic oxidation by using a fixed-bed multitubular reactor equipped with a heat-medium bath, a flow rate of the heat-medium is controlled so that a temperature of the heat-medium bath is raised to the extent of 2 to 10° C. in the course of the time that the heat medium is introduced in the heat-medium bath through the inlet part of the heat medium and moved to reach the outlet part of the heat medium.
Furthermore, as a method of measuring a temperature of the longitudinal direction of a reaction tube, for example, as disclosed in Japanese Patent Application, First Publication No. 2002-212127, a method of measuring the temperature of the longitudinal direction in the reaction tube, wherein some of the reaction tubes which represent the whole fixed-bed multitubular reactor are provided with protecting tubes before packing catalysts, into which thermocouples are inserted is exemplified.
However, any concrete methods to operate an adequately stable gas-phase catalytic oxidation reaction were not disclosed in the methods described in Japanese Patent Application, First Publication No. Hei 6-192144. That is, a concrete method for changing an activity or a concrete length of each section of a reaction tube to realize an operation for a stable reaction were not disclosed. Furthermore, in Japanese Patent Application, First Publication No. Hei 6-192144, an absolute value of the maximum value of ΔT (hereinafter, referred to as ΔT max or a hotspot part) in the first and the second sections was not disclosed and, in addition, because a difference between the ΔT max of the first and the second sections was large, it was difficult to perform an adequately stable operation.
Further, as a method described in Japanese Patent Application, First Publication No. 2001-139499 or Japanese Patent Application, First Publication No. Hei 8-92147, it is important for a stable and high-efficiency operation to maintain the optimum condition through monitoring a temperature of a catalyst layer, but in these literatures, only a method of measuring a quantity or a temperature of a heat medium which was flowed in a reactor shell side was disclosed and a technology to measure the temperature of the catalyst layer precisely was not disclosed. As a result, a position of ΔT max was not identified sufficiently and the reaction conditions were not maintained adequately stably.
Further, a method described in Japanese Patent Application, First Publication No. 2002-212127 was able to easily measure the temperatures of various positions in the longitudinal direction of a catalyst layer, however, it was too complicated to be practical for a commercial use in a fixed-bed multitubular reactor. That is, the fixed-bed multitubular reactor of an industrial scale mostly has hundreds to thousands of reaction tubes, and in many cases it has tens of thousands, and moreover, the length of the reaction tube is several meters, so that it was difficult to understand and control the temperature of the catalyst layer in all of the reaction tubes because the number of thermocouples was too large.
Further, a temperature was sometimes measured by selecting several reaction tubes out of a plurality of reaction tubes and inserting thermocouples into the selected reaction tubes, however, because measuring positions were not determined, it was insufficient to understand a position of the maximum temperature (ΔT max) in the longitudinal direction of the catalyst layer, which is most important for a stable operation.
The present invention has been achieved taking the above-mentioned circumstances into consideration and has an object to provide a fixed-bed multitubular reactor in which an oxidation reaction can be operated stably under the optimum condition with a supreme level by measuring a temperature distribution precisely and practically in the longitudinal direction of a reaction tube packed with a catalyst of the fixed-bed multitubular reactor and understanding a position of a hotspot part.